1. Field of the Invention
The present invention relates to the degradation compensation method for optical signals, and more particularly it relates to a device for compensating for the degradation of a signal waveform in an optical transmission line, and especially a polarization mode dispersion compensator for compensating for the degradation of an optical signal waveform, due to polarization mode dispersion in high-speed transmission or long-haul transmission.
2. Description of the Related Art
Lately, with the spread of the Internet, the transfer capacity of data using optical signals has increased, and a wavelength-division multiplexing (WDM) technology for realizing high-speed large-capacity communication has been introduced. However, for example, in an optical transmission system with a transfer rate of 40 Gbit/s or more, polarization mode dispersion (PMD) is one of important factors that restrict transmission coverage.
Since the core of a general single mode fiber (SMF) is not truly circular and slightly elliptical, birefringence is generated. An optical signal that is doubly refracted and inputted to a fiber is split into two orthogonal polarization mode components, that is, a fast-wave axis and a slow-wave axis. Since the polarization mode components differ in transfer rate, differential group delay (DGD) is caused. The differential group delay caused between modes after an optical signal goes through a birefringence medium including a fiber is called polarization mode dispersion.
Although a SMF with an ideally true circular core does not cause polarization mode dispersion, the core of an actual fiber causes slight distortion, that is, birefringence in its actual manufacturing process or due to a variety of stress (such as change in temperature, binding, twist, tension, etc.). PMD has almost no correlations with a wavelength and varies with the environmental change of a transmission line, such as temperature, stress or the like, as time elapses.
The PMD value of a laid fiber is proportional to the square root of distance. It is generally said that an old fiber laid overseas has a large PMD value per unit length, such that exceeds 0.2 to 2 ps/√km, and the transmission coverage in 40 Gbit/s transmission is restricted to 3 to 50 km (if it is assumed that the worst PMD value is three times as much as an average). For example, even in a system with a transfer rate of 10 Gbit/s, waveform degradation due to PMD cannot be neglected in super-long haul transmission of several thousands or more of kilometers, and in 40 bit/s or more super-high speed communication or a super-long haul distance system, an automatic PMD compensation technology for disposing a PMD compensator in the relay node of a transmission line is needed.
As such a prior art for compensating for the degradation of optical signals in a transmission line, Japanese Patent Laid-open Application Nos. 2001-53680 “Dispersion Compensator”, 2001-186084 “Optical Transmitting Terminal System for Optical Wavelength Division Multiplexing”, 2002-303805 “Variable-Dispersion Compensator and Optical Transmission System” and 2003-233045 “Polarization Control Device and Differential Delay Time Compensation Device” are disclosed.
In Japanese Patent Laid-open Application No. 2001-53680, if the wavelength dispersion of WDM optical signals covering a wide-range wavelength band is compensated for using a plurality of fiber gratings, the optical characteristic of an optical signal after compensation can be improved by setting so that each of a plurality of wavelengths obtained by demultiplexing input light may be the center frequency of the reflection wavelength of each of a plurality of dispersion compensation fiber gratings.
In Japanese Patent Laid-open Application No. 2001-186084, an optical communication terminal system is disclosed in which the number of devices can be reduced by partially sharing accumulated wavelength dispersion compensation devices provided for each channel in an optical transmitting terminal for optical wavelength division multiplexing.
In Japanese Patent Laid-open Application No. 2002-303805, a variable chromatic dispersion compensator is disclosed in which dispersion caused in signal light can be compensated for with high accuracy by using a movable mirror whose reflection position is variable, as a reflection mirror when reflecting each frequency component obtained as a result of demultiplexing signal light whose wavelength dispersion should be compensated for, by a corresponding reflection mirror and giving a predetermined phase shift.
In Japanese Patent Laid-open Application No. 2003-233045, a polarization dispersion compensation device is disclosed in which both a polarization control function to control a polarization state in order to compensate for polarization mode dispersion and a differential delay compensation function to compensate for differential group delay can be realized on one bulk device.
However, the technologies disclosed by Japanese Patent Laid-open Application Nos. 2001-53680, 2001-186084 and 2002-303805 do not compensate for polarization mode dispersion targeted by the present invention, and compensates for only simple-wavelength dispersion. Therefore, the technologies cannot compensate for polarization mode dispersion, which is a problem. The technology disclosed by Japanese Patent Laid-open Application No. 2003-233045 should be basically applied to an optical signal with a single wavelength, and if optical signals with a lot of wavelengths are multiplexed and transferred in a wavelength-division multiplexing method, a number equal to the number of the wavelengths, of polarization dispersion compensation devices are needed, which is another problem.
Next, the prior art of a polarization mode dispersion compensator is further described with reference to FIGS. 15 through 17. FIG. 15 is a block diagram showing the conventional configuration of a polarization mode dispersion compensator. In FIG. 15, optical signals are supplied from a transmission line 100 to a polarization controller 101. The polarization controller 101 adjusts the polarization state of each input optical signal and reshapes its optical waveform. The output is supplied to a DGD compensation unit 102. The DGD compensation unit 102 compensates for differential group delay by providing differential delay basically the reversal of the differential group delay caused in the transmission line 100 to compensate for the degradation of an optical signal waveform.
The output of the DGD compensation unit 102 is partially demultiplexed by a coupler 103 and is supplied to a PMD monitor 104. The PMD monitor 104 detects a polarization mode dispersion value contained in the output of the DGD compensation unit 102. The polarization mode dispersion is detected, for example, as a value indicating the degree of polarization (DOP) is supplied to a control circuit 105, and polarization mode dispersion detected by the PMD monitor 104 can be reduced by the control, circuit 105 controlling the polarization state of the polarization controller 101.
FIG. 16 explains polarization mode dispersion due to differential group delay (DGD). In FIG. 16, when an optical signal is inputted to an optical fiber, the optical signal is split into a fast wave axis component and a slow wave axis component by the differential group delay. The left side drawing shows both the waveform and combined vector of an optical signal when there is no DGD. When there is no DGD, the fast wave axis component and slow wave axis component have the same speed, and as a result, the combined vector faces the same direction at any time. This state corresponds to a state in which the degree of polarization is 100%. However, if there is DGD as shown in the right side drawing, the speed of the fast wave axis component and that of the slow wave axis component are different, and its combined vector faces a variety of directions depending on a clock time. As a result, its degree of polarization becomes less than 100%.
FIG. 17 is a block diagram showing the conventional configuration of a polarization mode dispersion compensator for input optical signals transmitted in a wavelength-division multiplexing method. In FIG. 17, an optical signal inputted from a transmission line is demultiplexed into multiplexed signals with each wavelength, by a demultiplexer 110 and each of the demultiplexed signals is inputted to a polarization controller 111. As in shown is FIG. 15, polarization mode dispersion compensation is applied to each wavelength component by a polarization controller 111, a DGD compensation unit 112 and a PMD monitor 114 (including a controller), and optical signals with each wavelength, after compensation, are multiplexed and outputted by a multiplexer 115.
However, in such prior arts or if the technology disclosed by Japanese Patent Laid-open Application No. 2003-233045 is used, in a wavelength-division multiplex transmission system, a PMD compensator must be disposed for each wavelength, that is, channel. For example, in FIG. 17, a polarization controller, a DGD compensation unit and a PMD monitor must be disposed for each wavelength, and, for example, the size and cost of a relay node in a transmission line, in which a polarization mode dispersion compensator is disposed, increases, which is a problem.